Did you make this instructable?

A mixture of iodine and starch is a dark blue-black colour. If you add a drop of this mixture to a liquid containing vitamin C, the vitamin C reacts with the iodine, consuming it. That means there is no iodine left to turn the starch blue-black, and the mixture loses its colour.

So, the de-colourisation of a drop of iodine/starch solution will be a test for vitamin C.

(I'll leave it to you and your son to find the appropriate proportions and quantities yourselves, but the googling will be easier if you just use the word "starch" rather than "cornstarch".)

I'm assuming Iodine and cornstarch make menthyl blue. Like Kiteman said you add it in drops in the orange juice to find out how much Vitamin C is in it. When the color stops changing, that means to stop. The more drops means the more Vitamin C! But, remember there is a fine line between the right amount and adding too much.